The present invention relates to the preparation of hydrofluorocarbons (HFC's). More particularly, the invention pertains to a method for the preparation of difluoromethane (HFC-32), 1,1,1-trifluoroethane (HFC-143a), 1,1,1,3,3-pentafluoropropane (HFC-245fa), 1,1,1,3,3,3-hexfluoropropane (HFC-236fa), 1,1,1,3,3-pentafluorobutane (HFC-365mfc) by reacting the corresponding hydrochlorocarbon feedstocks and HF in a liquid phase catalytic reactor using a large mole ratio of HF to hydrochlorocarbon to minimize formation of high boiling by-products and improve HF consumption and hydrofluorocarbon yields.
In recent years there has been widespread concern that chlorofluorocarbons might be detrimental to the Earth's ozone layer. As a result, there is a worldwide effort to use halocarbons which contain fewer chlorine substituents. In this regard, 1,1,1,3,3-pentafluoropropane and other HFC's have zero ozone depletion potential, and are being considered as a replacement for chlorofluorocarbons in many applications. The production of hydrofluorocarbons, i.e. compounds containing only carbon, hydrogen and fluorine has been the subject of interest to provide environmentally desirable products for use as solvents, foam blowing agents, refrigerants, cleaning agents, aerosol propellants, heat transfer media, dielectrics, fire extinguishing compositions and power cycle working fluids. It is known in the art to produce hydrofluorocarbons by reacting hydrogen fluoride with various hydrochlorocarbon compounds. Such HFC's are not only considered to be much more environmentally advantageous than hydrochlorofluorocarbons (HCFC's) or chlorofluorocarbons (CFC's) because they are non-ozone depleting, but also is they are also non-flammable and non-toxic as compared to the chlorine containing compounds. Hydrofluorocarbons are themselves well known. For example, HFC-245fa itself is well known in the art as described in U.S. Pat. No. 2,942,036, which is incorporated herein by reference. It has been a problem in the art to conduct an economical process for the continuous preparation of hydrofluorocarbons. One such process has been disclosed in U.S. Pat. No. 5,763,706, which is incorporated herein by reference. This reference shows reacting 1,1,1,3,3-pentachloropropane (HCC-240fa) and 1,1,1,3,3,3-hexachloropropane (HCC-230fa) with hydrogen fluoride in the presence of a fluorination catalyst, however, this process employs a relatively low molar ratio of hydrogen fluoride to HCC-240fa or HCC-230fa. This technique produces a disadvantageously large amounts of high boiling point by-products and a lower than desired catalyst life. It has now been found that by the use of a large HF to hydrochlorocarbon (organic feed) ratio, of at least about 15:1 in the liquid phase, the reaction can reduce high boiling point by-product formation and also prolong catalyst life. This is opposed to the prior art teaching to use organic-rich, HF-lean conditions.
It has now been found that hydrofluorocarbons, such as HFC-32, HFC-143a, HFC-245fa, HFC-236fa, HFC-365mfc, but not limited thereto, may be continuously and economically produced in an integrated manufacturing process by the reaction of hydrochlorocarbons with a high mole ratio of hydrogen fluoride. The hydrochlorocarbon and HF are first reacted, in either the liquid or vapor phase, but preferably in a liquid phase catalytic reaction, and a portion of the excess amounts of HF is optionally recycled back to the reactor such as by using a recycle column. HCl is then optionally removed by distillation, additional HF is recovered, such as by liquid-vapor or liquid-liquid extraction and then optionally recycled. Unsaturates are thereafter removed by photochlorination and hydrofluorocarbons are obtained by distillation.